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Abstract Socioecological theory predicts that male parenting among mammals should be rare due to the large payoffs of prioritizing mating effort over parenting. Although these predictions are generally met, in some promiscuous primate species males overcome this by identifying their offspring, and providing benefits such as protection and resource access. Mountain gorillas, which often organize into multi-male groups, are an intriguing exception. Males frequently affiliate with infants despite not discriminating their own from other males’ offspring, raising questions about the function of this behavior. Here we demonstrate that, independent of multiple controls for rank, age, and siring opportunities, male gorillas who affiliated more with all infants, not only their own, sired more offspring than males who affiliated less with young. Predictive margins indicate males in the top affiliation tertile can expect to sire approximately five times more infants than males in the bottom tertile, across the course of their reproductive careers. These findings establish a link between males’ fitness and their associations with infants in the absence of kin discrimination or high paternity certainty, and suggest a strategy by which selection could generate more involved male parenting among non-monogamous species. 

Abstract In humans and chimpanzees, most intraspecific killing occurs during coalitionary intergroup conflict. In the closely related genus Gorilla, such behavior has not been described. We report three cases of multi-male, multi-female wild mountain gorilla ( G. beringei ) groups attacking extra-group males. The behavior was strikingly similar to reports in chimpanzees, but was never observed in gorillas until after a demographic transition left ~25% of the population living in large social groups with multiple (3+) males. Resource competition is generally considered a motivator of great apes’ (including humans) violent intergroup conflict, but mountain gorillas are non-territorial herbivores with low feeding competition. While adult male gorillas have a defensible resource (i.e. females) and nursing/pregnant females are likely motivated to drive off potentially infanticidal intruders, the participation of others (e.g. juveniles, sub-adults, cycling females) is harder to explain. We speculate that the potential for severe group disruption when current alpha males are severely injured or killed may provide sufficient motivation when the costs to participants are low. These observations suggest that the gorilla population’s recent increase in multi-male groups facilitated the emergence of such behavior, and indicates social structure is a key predictor of coalitionary aggression even in the absence of meaningful resource stress.